If you want to do boosting, I'd have a higher toon unlinked.
That way they can join the group, and be as many levels above the four linked toons as you like.
In the "near" future, this will prevent bonus experience from RAF, if anyone in the group is high enough for the content to be trivial.

Of course "near future" has meant a day, and it has also meant over three years for Blizzard.
It is on the to do list, but probably not a high priority.